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from the in-star-trek-every-planet-is-habitable dept.
Earlier this month, a team of astronomers from the University of Liège, Belgium, and NASA, using the TRAPPIST instrument at the ESO site in La Silla, Chile, discovered three exoplanets with temperatures similar to those of the Earth and Venus orbiting an ultra-cool M class dwarf star. M dwarves makes up approximately 75 percent of the stars in our galaxy, and this discovery greatly improves the potential for habitable exoplanets.
A recent study from the Imperial College of London, however, suggests that, whilst these planets orbit cooler and less luminous stars, they may still be too hot to be habitable for life as we know it. Dr James Owen, Hubble Fellow and lead author of the study states:
It was previously assumed that planets with masses similar to Earth would be habitable simply because they were in the 'habitable zone'. However, when you consider how these planets evolve over billions of years this assumption turns out not to be true.
The atmospheres of these exoplanets are estimated to make up rougly one percent of their planetary mass. By comparison, the mass of our atmosphere is roughly one millionth of the mass of our planet. This suggests an especially thick atmosphere which, given the resultant greenhouse effect, may render these exoplanets uninhabitable. Naturally, this problem is inherent of planets of Earth-like mass, or heavier; smaller planets, e.g. of Mars-like mass, may lose some of this atmospheric mass through evaporation, as we have observed in our own solar system. Further cataloging of the orbital systems of M class dwarf stars will identify these low mass exoplanets as candidates for the ongoing search for extra-solar life.
(Score: 0) by Anonymous Coward on Friday May 27 2016, @08:54AM
Doesn't this end up increasing the size of the habitable zone?
(Score: 2) by b0ru on Friday May 27 2016, @09:12AM
The earlier study suggests that habitable zones may exist around M class dwarf stars, which increases the set of potentially habitable exoplanets. The latter study, however, posits this set of exoplanets may be diminished by effects of thicker atmospheres e.g. like the planet Venus. The habitable zone pertains to suitability for intelligent life as we know it but, ultimately, so-called 'extremophile' life may survive in far harsher conditions.
(Score: 2) by Immerman on Friday May 27 2016, @04:41PM
I think the point is - if you predict a strong greenhouse effect to be common, then that effectively just moves the habitable zone further from the star. If Mars and Venus were in opposite orbits they would both have far more mild temperatures.
(Score: 2) by Immerman on Friday May 27 2016, @05:08PM
Or at least just pushing it further from the star.
Having only read the abstract, and done a quick search for "greenhouse", there also seems to be some inconsistency in the paper. Perhaps someone with the patience to read more thoroughly can clarify: The abstract suggests that these planets would have far more massive atmospheres due to a mach larger H/He envelope during formation, which I assume would mean a primarily H/He atmosphere, which I wouldn't think would be a dramatic problem - since neither are greenhouse gasses they would if anything tend to be more opaque to incoming light than to escaping infrared/heat.
The discussion of greenhouse effect meanwhile seemed to be referencing a predominantly C02/H20/N2 atmosphere, which would seem to be at odds with the initial assumption unless there's some mechanism wherein a H/He atmosphere would encourage the presence of those other compounds. I can't think of any process except perhaps for water, as abundant hydrogen gas would tend to bond with any free oxygen. But even that doesn't necessarily translate to higher levels of atmospheric H20, as the vaporization temperature of water increases rapidly with pressure - for example at 100atm the boiling point of water is about 300C. (and since the freezing point remains roughly the same, that essentially triples the temperature range of liquid water)
The only mechanism I can think of is that simply by having such a massive atmosphere even very low partial pressures of the gasses would allow for extremely high absolute amounts. But with such dramatic density differences I would think that the atmosphere would tend to stratify, with hydrogen being concentrated in the upper atmosphere, largely shielded from the other volatile gasses by a layer of much denser helium beneath it.
(Score: 2) by deimtee on Friday May 27 2016, @06:08PM
It may be that it narrows the habitable zone. Far enough out, the gases condense on the surface, and the place freezes. Close enough to vaporise and with a thick atmosphere it shoots all the way into Venus greenhouse territory.
Pretty much any gas is a greenhouse gas for some spectrum. Isn't a red dwarf likely to have a much higher infrared/visible light ratio anyway?
No problem is insoluble, but at Ksp = 2.943×10−25 Mercury Sulphide comes close.